Our objective is to clarify the structural and functional organization of the mammalian trigeminal brainstem nuclear complex (TBNC), and to determine how this organization is altered by craniofacial injury. Due to inherent limitations in the techniques used in prior studies, there are gaps in our knowledge of structure-function-projection relationships for individual neurons in the normal adult TBNC. The experiments that we are proposing combine anatomical and physiological techniques to provide simultaneous morphological, physiological and connectivity data for individual TBNC neurons. Intracellular recording and horseradish peroxidase (HRP) injection techniques will be used in combination with receptive field mapping and antidromic and orthodromic stimulation techniques. In the trigeminal system only the primary afferent neurons have been subjected to such an analysis and strong structure-function correlations have been demonstrated. Given our knowledge of their afferent inputs, we can now undertake an analysis of higher-order trigeminal neurons. Attention will be paid to those neurons which process oro-facial nociceptive inputs, in an attempt to clarify the morphological substrates of trigeminal pain. TBNC neurons receiving vibrissae inputs will also be intensively studied, inasmuch as this trigeminal subsystem has long served as an effective model of central nervous system pattern formation and injury-induced reorganization. A second group of experiments will examine the number and distribution of TBNC neurons which project to more than one target via axon collaterals. Regrograde transport of 3 fluorescent dyes and HRP will be used in various combinations in individual animals to determine how oro-facial inputs are distributed to numerous functionally distinct central targets. A third group of experiments will test the hypothesis that functionally different trigeminothalamic neurons have morphologically distinct axon terminal arborizations in the thalamus. Medial lemniscal axons will be injected with HRP subsequent to determining their receptive field properties and TBNC subnuclear origin. Labelled terminals will be examined both in the light and electron microscope. The above studies will provide new information regarding the normal organization of the TBNC. For example, our PRELIMINARY STUDIES suggest that nociceptive and tactile inputs converge at all levels of the TBNC, and that an anatomical substrate exists for complex local processing. These experiments will also provide normative baseline data for our future studies of injury-induced reorganization in the TBNC.